In a known method of this type, which is utilised in the industrial processing of car-body parts, namely during the gluing of car-body sheet metal panels, a hardenable glue is sprayed with the help of a spray head onto the edge of the inner surface of an outer car-body sheet metal panel, which is supposed to be glued to an inner sheet metal panel. This is done by the glue being mixed with compressed air in the spray head and being then sprayed onto the edge of the car-body outer sheet metal panel. The width of the sprayed-on strip of glue depends decisively on the relationship between the amount of glue and the amount of compressed air. The inner sheet metal panel is subsequently placed onto the car-body outer sheet metal panel and the edge of the outer sheet metal panel, which edge is sprayed with the glue, is folded around the inner sheet metal panel. The hardening process can then be activated, for example, by heating this arrangement.
By spraying on the glue no undesired air pockets occur in the fold during the hardening operation, which air pockets would bring about a danger of corrosion. However, variations in the consistency of the strand of glue exiting from the spray head, or in the amount conveyed can occur during the spray application, which can result in changes in the width of the strip of glue sprayed onto the workpiece even with a constant compressed-gas supply. These variations can, for example, be due to a change in the viscosity of the glue, which in turn can be due to variations in the temperature or changes in the composition of the glue. This can result either in an incomplete filling of the areas to be glued or in protruding glue. Both cases have disadvantages. When the strip of glue is too narrow, the gluing of outer sheet metal panel to the inner sheet metal panel is insufficient, whereas when the strip of glue is too wide glue can protrude over the edge of the fold, which can lead to difficulties during the hardening operation and during the later cleaning operation.
Starting out from this, an object of the invention is to develop a method and a device of the above-mentioned type, with which an even application of glue onto the workpiece surface is guaranteed.
To attain this purpose the invention suggests that the amount of compressed gas supplied per unit of time and/or its flow speed is controlled in such a manner that a strip of paste of an essentially constant width is sprayed on along the workpiece.
In order to achieve this, it is possible according to an advantageous development of the invention to measure the feed pressure of the paste prior to its entry into the spray device and to control the amount of compressed gas supplied per unit of time in functional dependency of the measured feed pressure.
A further advantageous or alternative development of this method is to measure the temperature of the paste prior to its entry into the spray device and to control the amount of compressed air or inert gas in functional dependency of the measured temperature.
A further advantageous development of the invention provides that the volume or mass stream of the paste is adjusted to a desired value by varying the feed pressure at a constant flow cross section of the spray device. This is done, for example, in such a way that the feed or mass stream is adjusted to a predetermined value by varying the driving pressure or the electrical driving performance of a preferably electrically driven metering device. The compressed gas supplied per unit of time can in this case be controlled in functional dependency of the driving pressure or rather the driving performance or the driving voltage level derived therefrom. The stream volume can thereby be measured directly by means of a flow meter or indirectly through the piston speed of a metering cylinder forming the metering device. The functional dependency between the compressed-gas supply and the measuring parameter determining the compressed-gas supply can be provided, for example, in the form of a stored digital or analog, empirically determined calibration curve.
At varying mass flow through the spray device it is basically also possible to control the relative speed between spray device and workpiece in such a manner that the same amount of paste per unit area is sprayed onto the workpiece.
According to the device the object of the invention can be attained by providing a device for controlling the compressed-gas supply to the spray head, through which the compressed-gas supply is controlled in accordance with a desired width of the strip of glue applied along the workpiece.
The device of the invention includes a storage container for the paste, a feed pump for feeding the paste to a metering device, a further storage container or a supply connection for the compressed gas and a valve for controlling the compressed-gas supply to a spray head, with the spray head mixing the paste coming from the metering device with compressed gas and spraying it onto the workpiece, while the workpiece and the spray head are being moved relative to one another. A valve for the compressed-gas supply is in this case controlled through the control device in such a manner that the spray head sprays a strip of paste of an essentially constant width on along the workpiece.
An advantageous development of the invention provides a pressure sensor for measuring the feed pressure, which pressure sensor is arranged in front of the spray head, or a temperature sensor for measuring the temperature of the paste and a device for controlling the compressed-gas supply to the spray head, which device reacts to the measured feed pressure or the measured mass temperature.
A regulator is advantageously arranged in front of the spray head, through which regulator the volume or mass stream of the paste can be adjusted to a predetermined desired value by varying the feed pressure at a constant flow cross section in the spray head. In addition a preferably electrically driven metering pump for the paste, which metering pump can be controlled through the regulator, and a device for controlling the compressed-gas supply to the spray head, which device reacts to the driving pressure or rather the driving performance or voltage level of the metering pump, are provided. The volume or mass stream can be measured with the help of a flow meter arranged in front of the spray head in the feed pipe or with the help of a device for measuring the piston speed of a metering cylinder forming the metering pump, while the device for controlling the compressed-gas supply contains advantageously an analog or digital calibration-curve generator.
The measures of the invention give consideration to the knowledge that depending on the viscosity or stiffness of the glue a more or less viscous strand of glue exits from the application nozzle of the spray head. On the other hand, depending on the viscosity of this strand a more or less large amount of air or air speed is needed in order to selectively guide the strand in order to thus arrive at a specific strip width. The functional interrelations in this regard are given consideration according to the invention with the help of the empirically determined and electronically stored calibration curve.
The paste can be supplied through a metering cylinder to the application nozzle in the spray head. The piston speed is measured in this case as an indication of the volume of the amount of glue applied per unit of time. The driving pressure acting onto the piston is available as a further measurement. The pressure is higher at a pregiven metered amount when the viscosity of the paste is high. This pressure is thus an indirect measurement for the viscosity of the paste, which in turn depends on the temperature and on the composition of the paste. The driving pressure is usually produced through a feed system acting onto the piston, which feed system is, for example, electrically driven. The electrical voltage, which is applied to the feed system and which is essentially pressure-proportional, can thus be measured as a control variable for the feed control and thus as a stream volume control variable, and as a control variable for the compressed-gas supply to the spray head taking into consideration the previously-mentioned calibration curve. From a physical viewpoint this means that the voltage value measured at the feed system at a constantly adjusted mass rate of flow is a measurement for the stiffness or viscosity of the strand of mass exiting from the application nozzle.
In particular in the case of large technical applications, in which the paste is not supplied through a metering cylinder, but through an annular pipeline, which is tapped at individual stations, measuring the feed stream is necessary with the help of a flow-measuring device, for example a gear measuring cell or an inductive measuring cell in order to carry out control over a stream volume. Furthermore a pressure sensor arranged as much as possible near the application nozzle is additionally needed in this case, through which the compressed-gas supply (also in accordance with an empirical calibration curve) is controlled.